1. Field
The field relates to non-aqueous electrolyte secondary cells, and in particular a battery having a fast charge and discharge rate capability and low rate of capacity fade during such high rate cycling. The battery can exhibit low impedance growth, allowing for its use in hybrid electric vehicle applications and other high demand applications.
2. Description of the Related Art
Contemporary portable electronic appliances rely almost exclusively on rechargeable Li-ion batteries as the source of power. This has spurred a continuing effort to increase their energy storage capability, power capabilities, cycle life and safety characteristics, and decrease their cost. Lithium-ion battery or lithium ion cell refers to a rechargeable battery having an anode capable of storing a substantial amount of lithium at a lithium chemical potential above that of lithium metal.
Historically, non-aqueous secondary (rechargeable) cells using metallic lithium or its alloys as the negative electrode were the first rechargeable cells capable of generating high voltages and having high energy density. However, early on it became clear that their capacity decreased rapidly during cycling, and that their reliability and safety were impaired by the growth of the so-called mossy lithium and lithium dendrites to a degree that precluded these cells from the consumer market. Importantly, the few lithium-metal rechargeable batteries which, from time to time, were being actively marketed, were recommended to be charged at a rate no higher than ca. C/10 (10-hour) rate to minimize the dendritic growth.
To counteract the slow but unavoidable reaction of lithium with the electrolyte components, these early cells typically contained a 4-5 times excess of metallic lithium as compared with the capacity of the positive active material. Thus, the observed capacity fade during cycling was caused by a decrease in the specific capacity of the positive active material. A review of lithium-metal batteries is available (D. Aurbach et al., Journal of Electrochemical Society, 147(4) 1274-9 (2000)).
To overcome the difficulties associated with the use of lithium metal negative electrodes, several major improvements in battery materials were introduced. Various types of carbon capable of highly efficient and reversible intercalation of lithium at low potentials were used as the negative electrode to eliminate the growth of lithium dendrites. See, U.S. Pat. Nos. 4,423,125 and 4,615,959. Highly conductive liquid electrolytes have been developed, which are stable at both low and high potentials vs. lithium. See, U.S. Pat. No. 4,957,833. High-voltage, high-capacity positive electrode materials based on lithiated transition metal oxides, such as LiCoO2, LiMn2O4 and LiNiO2 have been developed. See, U.S. Pat. No. 4,302,518.
Since the electrochemical potential of lithium metal is only ca. 0.1 V lower than the potential of the fully lithiated graphitic carbon electrodes, LiC6, used in Li-ion batteries, both are strongly reducing towards any materials in contact with them, such as the polymer binder and the liquid electrolyte lithium salt solution. In particular, liquid electrolyte components react with both metallic lithium and lithiated carbon to form a metastable protective layer on the surface of the negative electrode materials, the so-called solid-electrolyte interface (SEI) (E. Peled, “Lithium Stability and Film Formation in Organic and Inorganic Electrolyte for Lithium Battery Systems”, in “Lithium Batteries”, J. P. Gabano, Ed., Academic Press, London, 1983; p. 43).
However, the process of SEI formation and its partial renewal during battery cycling and storage irreversibly consumes a fraction of the active lithium from the battery and results in a loss of capacity. This loss is readily visible when one compares the amount of charge used during the first charge and then the discharge of the battery, a so-called formation cycle. During the first charge cycle of a new Li-ion battery, the positive active material is oxidized and Li+ ions diffuse in the liquid electrolyte towards the carbon negative electrode, where they are reduced to Li0 and intercalated between the graphene layers of the carbon structure. A significant fraction of this first-reduced lithium, up to ca. 50%, but more typically between 5 and 15% of the intercalatable lithium, reacts to form the above-mentioned SEI. Clearly, the amount of Li available in the positive electrode material has to be less than the sum of lithium necessary for the formation of the SEI and the available lithium intercalation capacity of the carbon material. If the amount of lithium removed from the positive electrode material is greater than that sum, the excess lithium will be deposited, or plated, as metallic lithium on the external surfaces of the carbon particles. The plated lithium is in the form of a very reactive high-surface-area deposit, so-called ‘mossy lithium’, which will not only degrade the battery performance due to its high electrical impedance, but will also seriously compromise its safety.
Even if the lithium intercalation capacity of the carbon material is large enough to accommodate all of the lithium from the positive electrode material, it is possible to plate lithium if the charging is done too quickly.
Due to the strong possibility of lithium plating on the carbon anode during the high-rate charge, manufacturers of Li-ion batteries recommend that such batteries are charged at an equivalent current no greater than one time the nominal cell capacity (1 C) until the upper maximum charging voltage is reached, followed by a constant-current (taper) segment (http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_LiIon_Charging.pdf). In practice, the charging step lasts from 1.5 to 2.5 hours, which is too long for certain applications, such as battery-powered tools, certain electronic devices and electric vehicles.
Hybrid electric vehicles are a particularly demanding application for batteries. Hybrid electric vehicles are powered by an energy conversion unit (e.g., a combustion engine or fuel cell), and an energy storage device (e.g., batteries). Hybrid electric vehicles can have a parallel design, in which the energy conversion unit and an electric propulsion system powered by the batteries are connected directly to the vehicle's wheels. In such a design, the primary engine generally is used for highway driving, while the electric motor supplies power when the vehicle is moving at low speeds and during hill climbs, acceleration, and other high demand applications. Series designs are also employed, in which the primary engine is connected to a generator that produces electricity. The electricity charges the batteries, which drive an electric motor that powers the wheels.
The U.S. government has defined performance criteria for batteries to be used in hybrid electric vehicles. See, e.g., U.S. Department of Energy, FreedomCAR Battery Test Manual for Power-Assist Hybrid Electric Vehicles (October, 2003). For example, the battery should have a minimum pulse discharge power of 25 kW (for 10 seconds), a minimum peak regenerative pulse power of 20 kW (for 10 seconds), a total available energy of 300 Wh (at C1/1 rate), a cycle life of 300,000 cycles, and a calendar life of 15 years. Maximum weight, volume, and cost are also defined.
Designing lithium-ion batteries having sufficiently high power and sufficiently low impedance growth to meet the requirements for use in a hybrid electric vehicle has proved challenging. Impedance growth detracts from the useful life of a battery. The impedance of a battery grows over time as the battery ages and repeated charge and discharge cycles lead to degradation of the electrode materials. Impedance growth is increased at higher temperatures. Due to the long battery life required for hybrid electric vehicle applications, impedance growth becomes an important factor toward the end of battery life. For cells exhibiting typical impedance growth (e.g., 30-50% over 12 years), battery packs must be oversized, or provided initially with excess capacity, so that they can meet the performance requirements throughout the entire battery life. Oversizing helps reduce the stress on the battery in two ways: 1) it reduces the current or power each cell must deliver and 2) it allows for loss of power or performance, while still meeting the requirements at end-of-life. This oversizing disadvantageously adds to the weight, volume, and cost of the battery packs. Accordingly, Li-ion batteries exhibiting low impedance growth, in addition to high power, are desired for use in hybrid electric vehicles.